This invention relates to soft and hard propylene copolymers which have excellent balance between heat resistance, impact resistance and heat fusibility or sealability and are suitable for shaping into various shaped article such as films, sheet, bottles, injection molded articles, flat yarn and the like, and to a process for producing the same. More specifically, this invention relates to the same copolymers having a primary structure of increased nonuniformity with substantial improvement of the above described characteristics, and to a process for producing this type of copolymers.
The homopolymer of propylene has been widely used as a general purpose resin largely developed by taking advantage of its characteristics such as suitably high heat resistance and relatively good optical properties. However, this homopolymer has inherent defects and even some of its properties considered to be advantageous are not fully satisfactory. In order to eliminate these defects and disadvantages, a number of its copolymers with various comonomers, in particular with alpha (.alpha.)-olefins, have been studied.
Among studies of copolymers with .alpha.-olefins, studies involving copolymers with ethylene are the most numerous, and these studies are aimed at the improvement of characteristics of propylene homopolymer, such as impact-withstanding strength, transparency, heat fusibility and the like. Considerable success has been achieved in improving impact-withstanding strength, but other properties or propylene homopolymer are not always sufficiently improved.
Research has been carried out on the copolymers of propylene with straight chain .alpha.-olefin such as 1-butene, 1-hexene, 1-octene, and the like, and with branched chain .alpha.-olefins such as 4-methyl-1-pentene. When these comonomers are introduced at random into the polypropylene chain, they tend to disturb the crystal structure of polypropylene, and the disturbance greater than that caused by introducing ethylene. Therefore, these .alpha.-olefins have been considered more effective than ethylene for softening polypropylene or increasing the heat fusibility thereof. On the other hand, however, the relatively high heat resistance which is possessed by polypropylene is sacrificed.
As is clear from above, though the defects of propylene and the advantages thereof which are not always sufficient have been eliminated or improved by copolymerization with various .alpha.-olefins, the heat resistance of propylene has been generally degraded. Accordingly, if the improvement of heat resistance together with that of other characteristics can be realized, polypropylene will be able to enter many fields of use where it has been considered unusable. This will be very significant in industry.